This chapter is from the book

This chapter is from the book

The first
two
chapters
were
a little
brutal
on the
theory
of electronics,
so in
this
chapter
we are
going
to start
making
our
turn
to digital
electronics
and
a more
practical
hands-on "results" approach
rather
than
a lot
of theory.
That
is,
we are
going
to introduce
or re-introduce
a number
of components
and
see
how
to use
them
along
with
some
examples,
rather
than
analyze
them
as we
did
in the
previous
two
chapters.
At the
end
of this
chapter
you
should
feel
very
comfortable
with
basic
electronics,
components,
and
what
to use
them
for.
You
may
not
be able
to design
much
in the
way
of complete
analog
systems,
but
you
should
at least
be able
to follow
along
and
understand
the
general
workings
of anything
later
in the
book
that
has
to do
with
analog
design.
Considering
that,
here's
what
this
chapter
has
in store:

Basic
Mechanical Components

Capacitor
Models

Inductor
Models

Filter
Design

Diode
Models

Voltage
Regulations

Power
Supply Design

Introduction
to Transistors

Implementing
Digital Logic with Transistors

Clocking
Logic

3.1 Basic Mechanical Components

Although the majority of electronics are passive components like capacitors
or resistors, along with active components such as digital chips, you still need
basic inputs such as switches, and ways to adjust values. These kinds of
components aren’t that sexy, but they are necessary. Let’s take a
look at two classes of "mechanical" components: switches and
potentiometers.

3.1.1 Switches

Switches
come
in thousands
of sizes
and
shapes.
For
example, Figure
3.1 depicts
a
number
of
switches
from
a
the
popular
switch
manufacturer
SwitchCraft,
found
on
the
Internet
at
http://www.switchcraft.com/.
They also
make lots
of other
connectors
and other
cool parts.
In any
case,
the main
function
of switches
whether
they are
large
or small
is to
interrupt
or connect
a signal
current
flow,
just as
the wall
switch
in your
bedroom
turns
on a light.
There
are a
number
of types
of switches,
but they
can be
generally
categorized
into their
functionality
by means
of describing
the number
of contacts
or circuits
they have,
along
with the
number
of possible
positions.
So that
we have
a common
vocabulary,
here are
some terms
used in
switch
descriptions
to begin
with:

Poles—The number of switch contact sets that conduct
current.

Throw/Way—The number of conducting positions: For
single or double, "throw" is used; for three or more, "way" is
used.

Momentary—Switch returns to its normal position
when released. A spring is usually employed internally to accomplish
this.

Open—The switch is in the off position, contacts
not conducting.

Closed—The switch is in the on position, contacts
conducting; there may be several on positions.

3.1.1.1 Momentary Switches

The simplest switch is the momentary switch. Figure
3.2 shows a schematic
diagram of two variations: normally open and
normally closed. The normally open variation means
that when you release the switch plunger, the circuit is
open; when you press it, you
close the circuit. The normally closed variation is
the opposite: You press the switch to open the circuit. Figure
3.3 shows some
pictures of classic momentary switches. The red cap
usually means normally open, while the black cap means
normally closed. In addition to the generic names, there is a more technical way
of naming these switches. In Figure
3.4, you see that there is a single circuit
to be opened/closed and the switch has a single state (released/depressed); thus
these switches are classified as Single Pole Single
Throw or SPST. What if you wanted to
control two circuits at once with a single momentary switch? No problem; you
simply need a Double Pole Single Throw or
DPST. That means there are two circuits, but still
only one state for them to be in. Figure
3.5 illustrates the schematic symbol
for a DPST switch.

3.1.1.2 Slide Switches

The next type of switch is called a slide switch. Some common slide switches
are shown in Figure
3.6. The point of a slide switch is that it’s not
momentary, but stays in the position you leave it in. Figure
3.7 depicts the
schematic symbol for a slide switch. As you can see, there are three connections
on this particular switch; therefore, it can connect circuit A with B or B with
C, but not A with C. The symbol on top of the switch represents the metallic or
conductive slider. So this particular switch would be Single Pole
Double Throw or SPDT. If there were two
sets of contacts (two circuits) then we would have a Double Pole
Double Throw as shown in Figure
3.8.

3.1.1.3 DIP Switches

The word DIP switch stands for dual
in-line parallel in the context of a switch; however, people
sometimes will say "dual in-line package" as well. Figure
3.9illustrates the electrical symbol of a DIP and Figure
3.10 shows images of
common DIP switches. DIP switches are usually nothing more that N slide switches
in parallel; they are used in most cases to set hardware flags, control
settings, etc. Each one of the switches is generally a SPST slide switch. You
can find them in very small packages such as DIP2 all the way up DIP32. Common
sizes are DIP4 and DIP8. When we get to digital electronics, we will use DIP
switches all over the place; for example to set control lines that are sampled
by the hardware or memory system.

For example, say that we need 4 control lines to either be 0 or 1; these
control lines might select a memory chip or a port, who knows, but as a hardware
designer we want a way to use a switch to send four 0/1 signals. Figure
3.11illustrates a DIP4 circuit that does this. Referring to the circuit, there are 4
independent switch circuits, each tied to ground, and each has a resistor
connecting it to the system’s +5 power supply. All the switch circuits are
identical, so we need only analyze one such as S0. When S0 is open, there is a
path from the +5 supply to the output of S0, so any connection made to
S0’s port will see a +5; this equivalent circuit is shown in Figure
3.12.
A small current Iopen will flow in this case. As long as
Iopen is very small (a few milliamps or microamps), the voltage drop
over the resistor will be small and the port at S0 will have nearly +5 volts on
it (a HIGH or "1" in digital electronics). The amount of current that
flows depends on the resistor and the circuit that is being fed by the port. In
most cases with TTL (transistor transistor logic), the load current will be
milliamps, and with CMOS (Complementary Metal Oxide Semiconductor), the load
current will be nearly 0 or in micro amps. The point being—the current
Iopen is very small and we don’t need to worry too much about
it. Now, the second case is more interesting; let’s look at that.

When the S0 switch is open, the port at S0 is feed with a +5 volt, but when
we close the switch then we "short" S0 to ground. What happens now?
Referring to Figure
3.13, there is an equivalent circuit shown for this case.
There are two current branches, one from the +5 through the resistor to ground;
this current Iclosed will always be

Thus, this is one consideration when selecting R. The second current is the
"sink" current needed by the device being
driven to pull it down to a logic level 0; this is in the specification/data
sheet and we will learn it later, but again might be a few micro to milliamps,
but since we are shorting directly to ground, this current will always be
sinkable. Therefore, this switch with a passive resistor array creates a 4-bit
0/1 digital signal. The one confusing thing about the arrangement is that when
the switch is CLOSED or
"ON," the circuit creates a 0 (0V); when the
switch is OPEN or
"OFF" the circuit creates a 1 (+5V). This is
simply the way that it is designed.

CAUTION

You might be tempted to save the resistors and make the circuit work more
intuitively by connecting each switch to +5. When the switch is on, there is a
+5 or "1"; when the switch is off there is no signal, so a
"0"—WRONG! When the switch is off, you have no idea what is going
to happen; this is like an uninitialized variable in C/C++. You must set the
voltage yourself, otherwise the system will "float" to whatever. In
fact, it might even work until someone waves their hand over the circuit and
creates a capacitive coupling to ground! So always complete the design and make
sure all digital signals are set by you. We will get into this more later.

One last note: The configuration of resistors we just saw in the DIP switch
example is called "pull ups." They are named that way since they pull-up
the signals. There are also pull-downs; you can pull signals down to ground as
well of course. However, most designers prefer pulling signals up rather than
down, since a digital "1" takes less current sometimes to generate than
a digital "0", so a pull-up and short to ground is cleaner.

3.1.1.4 Rotary Switches

Rotary switches are used when you want to gate or send a signal from one (or
more) sources to one of N possible destinations (old TV sets used to have these
rotary switches, for example). Figure
3.14 illustrates a one pole, N-way rotary
switch. The single pole can be thought of as the "common" node (not
necessarily ground), and by rotating the mechanical switch you can send the
common signal to any one of the N-connections. Or you can use the switch
backward and think of connecting one of the N-ways to the common. For example,
when I was in high school, I made a little communications network with TVs and
audio for the school. They wanted to be able to send a TV signal and audio
signal to one of 5 rooms from a single source. Of course this is trivial with
one of these switches; assuming a common system ground for example, I sent the
composite video signal to one of five rooms using a circuit as shown in Figure
3.15. A similar circuit was used for audio. Of course, I put the whole circuit
in a box, labeled it really nice and added a 9V battery with some blinking
lights to make it look like it did something more complicated!

3.1.1.5 A Plethora of Switches

Although the switches we just covered are the most commonly used, there are
so many kinds of switches you could literally fill a 2,000 page book with them.
In other words, if you have a mechanical event that you want to detect with a
switch, there’s something; I guarantee it. Some examples of exotic
switching mechanisms are

Hall Effect switches that switch or detect
magnetic fields.

Mercury switches that turn on/off
based on orientation and a floating glob of conductive mercury.

Reedswitches that turn on/off based on the
application of a magnetic field that effects an internal permanent magnet inside
the switch.

Pressure switches that turn on/off based on air
pressure or vacuum.

3.1.2 Potentiometers

Potentiometers (POT) are nothing more than variable resistors. Figure
3.16shows the schematic symbol of one along with a few actual devices. Basically,
there is a knob or slide on the potentiometer that allows you to adjust the
resistance from 0 to the maximum value. These are VERY useful devices.
Internally, potentiometers work more or less by having the port contacts 1,2,3
connected to a piece of resistive material like carbon. When the slide or knob
is turned, the current path length through the resistive element is changed as
shown in Figure
3.17. Since the resistance of any material increases
proportionally to its length, by placing a contact at two ends of the resistive
material (contacts 1 and 3) and then placing a movable contact that makes a
frictional contact to the resistive element (contact 2), you can change the
resistance between contact 1 and 2 (call it R12) and between contacts 2 and 3
(call it R23) by moving the slide, knob, etc. Therefore, referring to Figure
3.17, the resistance of each branch or contact pair is simply

That is, the resistance is always divided between the two circuit legs, where
Re is the resistance in ohms of the element per unit distance. Of course, we
don’t really care about this much detail when using real potentiometers,
and simply look at the specs for final resistance. So if a potentiometer is
rated at 10K ohms then the resistance between contacts 1 and 3 is always 10K
ohms; however, the resistance between contacts 1,2, and 2,3 changes as the
potentiometer is adjusted. The specs of a potentiometer usually describe the
action of the potentiometer. For example, "1-turn" would mean that there
is a single 360 turn of the knob or dial that goes from 0–N. For example,
let’s say that we have a 1-turn POT with a resistance of 10K. Let’s
design a voltage divider with it that creates a 2.5V voltage at one of the
contacts. Starting with the circuit shown in Figure
3.18a, we see that in fact,
the POT can be used as a voltage divider; we also know from our study of voltage
dividers that the voltage drop over each resistor R1 and R2 is simply their
values divided by the sum multiplied by the source voltage.

Considering this, we simply need R1=R2=5.0K; this is easy! Just create the
circuit as shown in Figure
3.18b and turn the knob (or dial) to 50% or half a
turn, and that’s it. Of course, you need to set Vin to 5.0V (but I left it
as a variable to show you that Vin is irrelevant of the voltage divider’s
"action"). And the cool part is that you can adjust the voltage simply
by turning the knob, so you have created an adjustable voltage supply. Of
course, there’s the problem that you can only pull very little current
with the supply, since any load would alter the impedance too much. However, if
you were to use say a 100 ohm POT for the adjustment instead of 10K ohm and then
only attach loads that pulled a few milliamperes, you would be fine.

As a final note, potentiometers are mechanical and thus have a lifetime; you
can’t turn them back and forth an infinite number of times. They are fine
for adjustments, but if you used them to constantly change something they would
wear out and break. Also, they are nothing more than resistors internally, and
thus have maximum power dissipation specifications in the watt range
usually.